The present invention relates to a sweeping process for a mass spectrometer that provides a mass analysis by collision induced dissociation or a so-called metastable ion spectrum method.
According to the collision induced dissociation method, sample ions are caused to collide with neutral molecules in a collision chamber that is disposed in the path in which the ions travel, in order to dissociate the ions. Then, spectra are obtained from the resulting daughter ions. According to the metastable ion spectrum method, the metastable ions from sample ions resolve themselves into smaller fragment particles in the Field Tree Drift Region without collision gas, resulting in daughter ions, from which spectra are derived. Both methods have evolved as useful tools for structural analysis of organic compounds or for the study of fragmentation of organic compounds.
To utilize either the collision induced dissociation or the metastable ion spectrum method, a MS/MS instrument is often employed. In this instrument, mass spectrometers are disposed before and after a collision chamber. The present inventor has already proposed a mass spectrometer taking the form of such an MS/MS instrument and in which a superimposed-field mass spectrometric unit constitutes the latter stage of the spectrometer (see U.S. Pat. No. 4,521,687). The structure of this proposed instrument is shown in FIG. 1(a). FIG. 1(b) is a cross-sectional view taken along the line A--A'. In these figures, an ion source 1, an electric field 2, and a magnetic field 3 are arranged in a conventional manner to constitute a double-focusing mass spectrometric unit. This first unit forms a point at which ions are converged, and a collision chamber 5 is located at this point. Disposed between the chamber 5 and a collector 4 is a second mass spectrometric unit having superimposed fields. Specifically, the second unit comprises magnetic pole pieces 6a and 6b for producing a magnetic field in the direction perpendicular to the page, a magnetic field power supply 7 for energizing the pole pieces, a pair of electrodes 8a and 8b for producing a toroidal electric field in the direction perpendicular to the magnetic field, an electric field power supply 9 for generating a voltage applied between the electrodes, auxiliary electrodes 10a and 10b, known as Matsuda plates, mounted between the magnetic pole pieces 6a and 6b on both sides of the toroidal field, and an auxiliary power supply 11 for applying a correcting voltage across the auxiliary electrodes. In this mass spectrometer employing the superimposed fields, the intensity of the magnetic field of the superimposed fields is switched between two levels, and at each of these levels the electric field is swept.